Eagerly do boundscheck when indexing into CartesianIndices (#42119)

Author: johnnychen94

base/multidimensional.jl

@@ -357,8 +357,14 @@ module IteratorsMD
     @propagate_inbounds function Base.getindex(iter::CartesianIndices{0,R}) where {R}
         CartesianIndex()
     end
-    @propagate_inbounds function Base.getindex(iter::CartesianIndices{N,R}, I::Vararg{Int, N}) where {N,R}
-        CartesianIndex(getindex.(iter.indices, I))
+    @inline function Base.getindex(iter::CartesianIndices{N,R}, I::Vararg{Int, N}) where {N,R}
+        # Eagerly do boundscheck before calculating each item of the CartesianIndex so that
+        # we can pass `@inbounds` hint to inside the map and generates more efficient SIMD codes (#42115)
+        @boundscheck checkbounds(iter, I...)
+        index = map(iter.indices, I) do r, i
+            @inbounds getindex(r, i)
+        end
+        CartesianIndex(index)
     end
 
     # CartesianIndices act as a multidimensional range, so cartesian indexing of CartesianIndices

test/boundscheck_exec.jl

@@ -259,4 +259,17 @@ if bc_opt == bc_default || bc_opt == bc_off
     @test !occursin("arrayref(true", typed_40281)
 end
 
+@testset "pass inbounds meta to getindex on CartesianIndices (#42115)" begin
+    @inline getindex_42115(r, i, j) = @inbounds getindex(r, i, j)
+
+    R = CartesianIndices((5, 5))
+    if bc_opt == bc_on
+        @test_throws BoundsError getindex_42115(R, -1, -1)
+        @test_throws BoundsError getindex_42115(R, 1, -1)
+    else
+        @test getindex_42115(R, -1, -1) == CartesianIndex(-1, -1)
+        @test getindex_42115(R, 1, -1) == CartesianIndex(1, -1)
+    end
+end
+
 end